Electronic flash units fire virtually instantaneously and reach full brightness immediately. Therefore no time delay is required. Also refer as X sync. Real time setting that causes the flash to burst in synchronises or instantaneously as the shuttle open up. For older manual camera, the X synch speed usually refers to the maximum speed that the camera can have its shuttle curtain open long enough to synchronise with the flash. In fact, if there is a time delay, the electronic flash may be all over before the shutter gets open. To fire electronic flash with a focal-plane shutter, the switch in the camera is closed at the instant the first curtain of the focal-plane shutter reaches fully open-called X synchronization.

X Sync Terminal

Electronic flash units are available which mount on the hot shoe and are triggered by the electrical contact in the shoe. Other types use sync cord which connects to the sync terminal on the camera. also referred to PC Terminal section.